The following studies are planned: 1. Characterization of the thyroidal enzymatic deiodination of iodotyrosine, with special reference to isolation of the presumed low redox potential electron carrier mediating reduction of the flavoenzyme moiety (dithionite-dependent deiodinase) by NADPH. 2. Continue attempts to purify the enzyme catalyzing 5' -monodeiodination of thyroxine, using cholate-solubilized enzyme derived from rat kidney microsomes. Special attention will be devoted to (a) possible lipid dependency of the soluble enzyme, (b) mechanism of activation by thiols and inhibition by propylthiouracil and reverse T3, and (c) reversal of the PTU-inhibition by thiols and thioureylene compounds. 3. Studies of thyroid-hormone dependence of the responsiveness of adipocytes to lipolytic hormones will be continued, using adipocytes from eu-, hypo- and hyperthyroid rats with special reference to variation in cellular level of phosphodiesterase activity and possible correlations with (a) alteration in calcium kinetics, as revealed by influx and efflux study with 45Ca ion2, (b) content of Ca-dependent phosphodiesterase activator protein in the adipocytes, (c) measurement of Ca ions-ATPase in subcellular fractions of the adipocytes. 4. Continuing studies in the inhibition of thyroid organic iodine formation by excess iodide, in vivo (rat) and in vitro (bovine thyroid slices) with special reference to correlations of organic binding capability with iodide-dependent inhibition of thyroid TSH-dependent adenylate cyclase.